This invention relates to a transistor amplifier circuit using complementary type transistors.
By convention, when a high impedance signal source is connected to a load circuit, an impedance converter is provided between the signal source and the load circuit, with an intention of reducing the impedance. Such impedance converter is required to satisfy a severe condition that the input impedance must be high and the distortion factor must be low over the wide range from low frequency to high frequency of the input signal. One of the widely used impedance converters is an transistor amplifier.
FIG. 1 shows an example of the conventional transistor amplifier circuits. In the figure, reference numerals 2, 4 and 6 designate an input signal source, an NPN transistor for amplification, and a load resistor, respectively. +Vcc and -Vcc denote a positive power source and a negative power source, respectively. Further, IN and OUT are an input and an output terminals, respectively. Ccb is a capacitance between the collector and base of the transistor 4. With designation that .beta. is used for representing the current amplification factor of the transistor 4 and R for the resistance of the load resistor 6, the input impedance is given by the product of the current amplification factor .beta. and the resistance R, i.e. .beta..multidot.R, in the low frequency region of the input signal. From this fact, it will be seen that increase of the input impedance is easily attained by using a transistor having a high current amplification factor .beta. and a load resistor having a high resistance R. In the high frequency region, however, the capacitance Ccb between the collector and base of the transistor 4 reduces the impedance therebetween. Therefore, in this case, it is difficult to keep the input impedance high.
For avoiding such problem, an amplifier circuit as shown in FIG. 2 was proposed. As shown, an amplifier 16 with voltage gain of about 1 is additionally inserted between the emitter and the collector of the transistor 12. With such a construction, the collector potential of the transistor is substantially equal to the emitter potential. In other words, the collector potential changes following the change of the input signal applied to the base. For this, the voltage between the collector and the base is kept substantially constant. Accordingly, the influence of the capacitance Ccb is reduced, this enabling the input impedance to be kept high even in the high frequency region. In FIG. 2, the load resistor is designated by reference numeral 14.
Let consider now the total harmonic distortion factor of this amplifier circuit. Even if the amplifier is used which satisfies the bias condition placing the transistor 12 in the active region and has a small distortion factor, since the characteristic of the base-emitter voltage versus emitter current of the transistor 12 is inherently nonlinear, it is difficult to reduce the distortion factor of the entire transistor amplifier circuit. In other words, the low distortion of only the amplifier 16 fails to improve the distortion factor of the entire transistor amplifier circuit.
FIG. 3 shows the variations of the total harmonic distortion factor when the frequency of the input signal is changed. The respective curves plotted in the graph result from the experiment conducted under the condition that the signal source 2 has the impedance of 48 (K.OMEGA.), the input signal is 3V, and the voltages of +Vcc and -Vcc power source are 10V. The distortion factor in the circuit of FIG. 2 is denoted by the character A. The distortion factor A is substantially constant independently of the frequency change, although its magnitude is high. This graph indicates that not only the distortion factor of the amplifier 16 but also the non-linearity characteristic of the base emitter voltage versus emitter current of the transistor 12 must be taken into consideration in order to improve the distortion factor of the entire transistor amplifier circuit. The second order distortion occupies most of the distortion due to the non-linearity inherent to the transistor. To cope with this problem a transistor amplifier circuit was proposed as shown in FIG. 4. As shown, in this amplifier circuit, complementary transistors 22 and 24 are connected in cascade fashion so that the second order distortions of the respective transistors can cancel to each other, resulting in reduction of the distortion factor. However, as in the FIG. 1 case, in the high frequency region, the influence of the non-linear capacitor Ccb between the collector and the base is actualized to reduce the input impedance and therefore to result in increase of the distortion factor. In the figure, reference numerals 26 and 28 designate first and second load resistors, respectively. The curve B in FIG. 3 shows the characteristic of the distortion factor versus to frequency in this circuit. As seen from the curve B, the distortion factor of the FIG. 4 circuit exhibits a low value in the low frequency range but a high value in the high frequency range.